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The Development of the Amazonian Mega-Wetland (Miocene; Brazil, Colombia, Peru, Bolivia)
No full textThe scenery of Western Amazonia once consisted of fluvial systems that originated on the Amazonian Craton and were directed towards the sub-Andean zone and the Caribbean. In the course of the Early Miocene these fluvial systems were largely replaced by lakes, swamps, tidal channels and marginal marine embayments, forming a mega-wetland. In this chapter we will review the characteristics of this mega-wetland and its different phases of development. These aquatic environments hosted a diverse fauna whereas the shores of these systems were fringed by palm swamps, and a diverse rainforest occurred in the peripheral dry lands. The genesis of this wetland was primarily driven by geological mechanisms such as the Andean uplift, and an increase in accommodation space in the sub-Andean and intracratonic basins. Additionally, high precipitation rates also played an important role in wetland formation. The earliest phase of wetland development is recorded in boreholes drilled in the sub-Andean foreland basins of Peru and Colombia, and in the intracratonic Solimões Basin of western Brazil. During the latest Oligocene to Early Miocene (~24 to 16 Ma) lacustrine conditions alternated with episodes of Andean and cratonic fluvial drainage as well as marginal marine influence. In Amazonia, marine incursions are intercalated as thin beds in the Middle to Upper Miocene fluvial strata and contain marine and coastal taxa (foraminifera, mangrove pollen). Lacustrine conditions expanded further during the Middle Miocene to early Late Miocene (~16 to 11.3 Ma; Pebas phase). During this period the lake-embayment and swamp systems - fringed by forested lowland - reached their maximum extension. This wetland was subject to marginal marine influence and sustained a large radiation of endemic aquatic invertebrate faunas. During its maximum extent the wetland covered an area of more than 1.5 × 106 km2 - comprising much of the Present western Amazonian lowlands. From the Late Miocene onwards uplift rates in the Eastern Cordillera, Cordillera Real and Cordillera de Merida substantially increased and the Andes became a continuous barrier. This barrier effectively separated lowland Amazonia from Orinoquia and the Magdalena Valley and closed off all lowland connections with the Pacific and the Caribbean. The wetland system became a complex environment where deltaic, estuarine and fluvial environments coexisted. This Late Miocene fluvial-tidal-dominated wetland (~11.3 to 7 Ma, Acre phase) hosted a species-rich vertebrate fauna, but (in contrast to the Pebas phase), the molluscan fauna was species poor and already strongly resembled the modern Amazonian fluvial fauna. This system represents the onset of the transcontinental Amazon River. From 11.3 Ma onwards, sediments of Andean origin reached the Atlantic continental shelf and initiated the build-up of the Amazon Fan
On the origin of Amazonian landscapes and biodiversity: a synthesis
No full textIn northern South America the Cenozoic was a period of intense tectonic and climatic interaction that resulted in a dynamic Amazonian landscape dominated by lowlands with local and shield-derived rivers. These drainage systems constantly changed shape and size. During the entire Cenozoic, the Brazilian and Guiana Shields were stable mountainous areas. Andean-derived river systems increased in importance especially in the Neogene. A remarkable feature in western Amazonian history is the waxing and waning of large lake systems and embayments. By the Late Miocene (about 11 Ma), the Andes were connected with the Atlantic through an incipient Amazon River, and from c. 7 Ma Andean-derived river systems became fully established in central and eastern Amazonia and the modern landscape configuration had developed. Rainforests already existed in northern South America during the Paleogene, but the modern rainforests - with resemblance to the Present forest - only developed during the Miocene. The western Amazonian Miocene record contains very diverse aquatic faunas (molluscs, ostracods, turtles, crocodiles, fishes) as well as terrestrial mammals. Remarkable gigantic forms thrived in Amazonian ecosystems at the time. Since the Late Miocene, edaphically heterogeneous lands emerged in western Amazonia in areas previously occupied by lake systems. At the same time nutrient-rich deposits spread over central and eastern Amazonia, an event that, based on molecular phylogenetic studies on extant taxa, coincided with diversification of terrestrial taxa. Molecular-based time estimates confirm the steady diversification and mostly pre-Quaternary origin of extant Amazonian taxa. A significant portion of the current species richness is attributed to a combination of relatively constant wet and warm climates and a heterogeneous edaphic substrate. The Quaternary was a time of distribution shifts, but can no longer be considered a time of diversification in Amazonia
The Amazonian Craton and its influence on past fluvial systems (Mesozoic-Cenozoic, Amazonia)
No full textThe Amazonian Craton is an old geological feature of Archaean/Proterozoic age that has determined the character of fluvial systems in Amazonia throughout most of its past. This situation radically changed during the Cenozoic, when uplift of the Andes reshaped the relief and drainage patterns of northern South America. Here we review the sedimentary characteristics of Amazonian rivers and compare these with four fluvial depositional settings from the Meso-Cenozoic sedimentary record. These sedimentary units are the Alter do Chão Formation (Brazil, Late Cretaceous-Paleogene), the Petaca Formation (Bolivia, Late Oligocene to Middle Miocene), the Mariñame and Apaporis Sand Units (Colombia, Miocene), and the Iquitos White Sand Unit (Peru, Late Miocene-Pliocene). This review illustrates that the river systems born on the craton share features such as sediment texture and composition, depositional environments and transport directions. Evidence for the diminished role of cratonic fluvial systems and the onset of Neogene Andean uplift can be identified in the sedimentary record by changes in sediment provenance and transport directions. Although the Andean uplift and related processes discontinued the major Amazonian-born fluvial systems it also created new topographic features such as the Iquitos and Fitzcarrald Arches. These newly formed reliefs triggered a new generation of rivers, some of which are presently known as biodiversity hotspots
The origin of the modern Amazon rainforest: implications of the palynological and palaeobotanical record
No full textNorthern South America harbours a highly diversified forest vegetation. However, it is not clear when this remarkable diversity was attained and how it was produced. Is the high diversity the product of a positive speciation-extinction balance that accumulated species over long time periods, or is it the product of high origination rates over short time periods, or both? Middle Cretaceous floras, although very poorly studied, are dominated by non-angiosperm taxa. By the Paleocene, pollen and macrobotanical fossils suggest that the basic phylogenetic composition and floral physiognomy of Neotropical rainforests were already present. Hence there was a profound change in Amazonian flora during the Late Cretaceous, that still needs to be documented. Levels of Paleocene diversity are much lower than those of modern tropical rainforests. By the Early Eocene, however, pollen diversity was very high, exceeding values of modern rainforests. At the Eocene-Oligocene a major drop in diversity coincided with an episode of global cooling. The palynological and palaeobotanical records of Amazonia suggest that high levels of diversity existed during the Miocene, a period when the boundary conditions for sustaining a rainforest (e.g. low seasonality, high precipitation, edaphic heterogeneous substrate) were met. The predecessor of the present rainforest was formed during the Paleogene and Neogene when the western Amazon lowlands were affected by Andean tectonism, which radically changed drainage systems and promoted wetland development. An overall global cooling during the Neogene also may have affected the rainforest, decreasing its area and expanding adjacent savanna belts. Recent events like the Quaternary ice ages also played a role in the forest dynamics and composition, although it seems to have been minor. In this chapter we will review the main characteristics of the Neogene palynological and palaeobotanical records in Amazonia, and we will make some comparisons with pre- and post-Neogene records. The data indicate that the Amazonian rainforest is more likely to be a product of a dynamic geological history stretching back over the past 25 million years rather than the last few hundred thousand years
Composition and diversity of northwestern Amazonian rainforests in a geoecological context
No full textThe northwestern Amazonian landscape includes most of the representative landscape units that characterize Amazonia, and for this reason it constitutes an excellent place to investigate relationships between the abiotic environment (geology, geomorphology, soils) and biodiversity. In this review we assess these relationships on three temporal/spatial scales. At the macroscale, the geology of this area includes the sub-Andean zone in the west, the Guiana Shield to the north, and the varied Neogene substrate and river valleys. These geological units account for much of the habitat variation that can be linked to species distributions. For instance, observations of different tree species communities in Ecuadorian or Peruvian Amazonia are explained by variations in soils and geological substrate. Mesoscale abiotic heterogeneity is reflected in the spatial configuration of different land units, which create conditions for habitat-specific forest types. Experimental field studies arc needed to compare effects of the abiotic environment' (water and soil nutrients) with those related to dispersal and species interactions. At micro scales, processes in the biotic environment probably contribute most strongly to pattern formation in diversity and composition of floras or plant communities
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